An Ultrastable π–π Stacked Porous Organic Molecular Framework as a Crystalline Sponge for Rapid Molecular Structure Determination

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022An Ultrastable π–π Stacked Porous Organic Molecular Framework as a Crystalline Sponge for Rapid Molecular Structure Determination Cheng Chen, Zhengyi Di, Hao Li, Jinying Liu, Mingyan Wu and Maochun Hong Cheng Chen State Key Lab of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 , Zhengyi Di State Key Lab of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , Hao Li State Key Lab of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , Jinying Liu State Key Lab of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , Mingyan Wu *Corresponding author: E-mail Address: [email protected] State Key Lab of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 and Maochun Hong State Key Lab of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 https://doi.org/10.31635/ccschem.021.202100910 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The crystalline sponge method is a pragmatic and promising strategy for molecular structure determination. However, the dominant metal–organic framework crystal sponge platforms always face poor chemical stability, especially solvent instability, hampering their application in a vaster domain. Herein, we report an ultrastable π–π stacked porous organic molecular framework which exhibits permanent porosity, high thermal stability, and good chemical resistance. It can efficiently implement an approach to molecular structure determination via a single-crystal-to-single-crystal transformation. This is the first example utilizing π–π stacked porous organic molecular framework as "crystalline sponge" to determine a wide variety of guests, ranging from hydrophilic to hydrophobic, and from aliphatic to aromatic, which complements the crystalline sponges based on the famous metal–organic frameworks. More importantly, it can achieve rapid structure determination of small molecules within 3 h. Download figure Download PowerPoint Introduction The single-crystal X-ray diffraction (XRD) technique provides definite structural information at the atomic level and is recognized as the most reliable method to structure determination. However, many organic molecules tend to form oils or amorphous phases or be frustrated while growing proper-sized single crystals with good crystallographic quality for XRD analysis. To obtain suitable crystals for analysis, Richert and co-workers1 have used the "adamantane crystallization chaperone" strategy to solve the crystallization problem of small molecules, Ward and colleagues2 employed similar tactics that relied on single-step cocrystallization of guest molecules within guanidinium organosulfonate hydrogen-bonded frameworks, while others have tried to make crystals growing easier by using polyelectrolyte solutions and shear flow,3 or employing the "encapsulated nanodroplet crystallization methodology" with smearing silicone oil upon analyte solution.4 Alternatively, Fujita and colleagues5–7 took a different tact to put forward an innovative protocol of the "crystalline sponge method," where the analytes do not require growth of single crystals to conveniently determine the structures of organic molecules through in situ single-crystal-to-single-crystal (SC-SC) process. This latter strategy perfectly solves the structure determination problem for hardly crystallized liquids and other stubborn noncrystallizing compounds,8,9 enabling absolute configuration confirmation of natural products,10–14 as well as inspection of transient intermediates in a chemical reaction.15,16 Until now, only a few cases of metal–organic frameworks (MOFs)5,9,17–27 have been proved for crystalline sponge platforms. Typical strategies based on MOFs platforms rely primarily on weak interactions5 or coordinative substitution21 to induce crystalline order of the target molecules. However, there are still some flaws that affect the expanded applicability. Although the pore features of MOFs can be designed reasonably, the versatile crystalline sponge hosts which can encapsulate different guests with hydrophilic, hydrophobic, aromatic, or aliphatic properties are seldom.5 On the other hand, Yaghi and co-workers21,25 have reported an unprecedented chiral MOF (MOF-520), which can utilize inventive competition coordination binding approach to adsorb target guests. To get strong interactions, however, the guests are required to possess special functional groups such as those found in primary alcohol, phenol, carboxylic acid, nitrogen-containing azolates, sulfur-containing oxoacids, and phosphorus-containing oxoacids.21,25 In addition, MOFs have poor stability in many solvents, especially in water, thereby limiting the host to identify more types of organic molecules. Thus, if the typical MOF hosts can improve the stability of crystal structure in more organic solvents, the detected scope of guest molecules can be further extended.5,28,29 In fact, many MOFs are designed with extremely high symmetry, suitable for gas adsorption. Especially, some flexible frameworks show interestingly gas-loaded SC-SC transformations under ambient conditions.30–33 Therefore, developing new classes of crystalline sponge candidates with both high stability and versatility that can record the information of guest structure accurately is highly desirable and yet remains a significant challenge. Noncovalent π–π interactions involving aromatic functional groups are critical for building large and complicated molecular architectures.34–45 Actually, the π–π stacking interactions are important driving forces for some rigid and stable organic porous materials, such as the two-dimensional (2D) covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs), which usually contain large π-conjugate aromatic groups.46 Introduction of large π-conjugate aromatic moieties as building skeleton enhances the chemical resistance for organic solvents, acids, and bases substantially, owing to their inert reactivity. Likewise, strong noncovalent π–π interactions can endow the supramolecular frameworks superior thermoelasticity, flexibility, and self-healing ability, as well as high fragile resistance when the crystals suffer touch, movement, or solvent cleansing.37,38,46 Herein, we demonstrate a porous organic molecular framework 1 constructed by noncovalent π-interactions as a new crystalline sponge platform, in which a series of target molecules can be encapsulated via an SC-SC process. The aromatic rings are orderly piled by parallel displacement of π stacking fashions to form a one-dimensional (1D) channel, which endows 1 efficiently for trapping typical aromatic and aliphatic compounds, as well as hydrophilic molecules through very strong host–guest interactions (C–H⋯π and C–H⋯O interactions). More importantly, we can unambiguously determine the guest molecule structure within 3 h. Experimental Methods Synthesis of APT 1,3,5-tris(4-bromophenyl)benzene (1.00 g, 1.85 mmol), 9-anthraceneboronic acid (1.90 g, 8.50 mmol), cesium carbonate (2.86 g, 8.80 mmol), and tetrakis(triphenylphosphine)palladium (0.20 g, 0.21 mmol) were added to a Schleck flask (250 mL) charged with a stirring bar. The flask was pumped under vacuum and refilled with argon three times, and then 150 mL of degassed 1,4-dioxane solvent was transferred to the system. The solution was heated at 90 °C for 72 h under an argon atmosphere ( Supporting Information Scheme S1). After the reaction mixture was cooled to room temperature, the organic solvent was evaporated. The crude product was washed with ethanol (3 × 200 mL) and distilled water (3 × 200 mL). Then, the crude product was recrystallized using tetrahydrofuran (THF) twice to give compound 9,9′-(5′-(4-(anthracen-9-yl)phenyl)-[1,1′:3′,1″-terphenyl]-4,4″-diyl)dianthracene ( APT) as a gray-white solid (1.08 g, 70% yield based on 1,3,5-tris(4-bromophenyl)benzene). The compound was characterized by proton nuclear magnetic resonance (1H NMR; 400 MHz, CDCl3, δ): 8.57 (s, 3H), 8.24 (s, 3H), 8.11 (t, 12H), 7.86 (d, 6H), 7.66 (d, 6H), 7.53 (t, 6H), 7.45 (t, 6H). Synthesis of 1 and 1-None The ligand APT (30 mg, 0.07 mmol) was dissolved in CHCl3 (2 mL), 1,4-dioxane (2 mL), and ethanol (2 mL). The mixture was placed in the Teflon vessel of the hydrothermal bomb and stirred for 5 min; then, the vessel was sealed, placed in an oven, and heated at 120 °C for 2 days. The reaction mixture was allowed to cool slowly to room temperature within 4 days. Light brown block crystals 1 were obtained in 66% yield. The as-synthesized crystals of 1 (100 mg) were immersed in ethanol solvent (10 mL) at room temperature for 3 days to obtain the guest-free crystals 1-None. Elemental analysis calc. for solvent-free 1-None (C66H42): C, 94.93; H, 5.07; found: C, 95.05/95.09%; H, 5.10/5.05%. Results and Discussion Crystal structure and the pore architecture Organic building block APT is a C3v-symmetric molecule with nonplanar propeller-like conformation. It consisted exclusively of nonpolar benzene (BZ) and anthracene rings, connected via single bonds (Figure 1a and Supporting Information Scheme S1). APT could self-assemble into moderate-sized single crystals ( 1) suitable for X-ray analysis. The as-synthesized crystal 1 adopted a low-symmetric space group P21/c and extended into a distinctive three-dimensional (3D) framework only by noncovalent π-interactions. The nonplanarity of this multijoint skeleton stemmed from a marked sterically hindered effect between the centered aromatic nucleus, the peripheral phenylene, and the anthracene rings. As expected, none of the three propeller-like wedges is identical, the connected aromatic rings adopting a dihedral angle in the range of 20.2–88.6° (Figure 1b and Supporting Information Figure S1). Despite many aromatic rings, only one pair of anthracene rings assumed a parallel-displaced π-stacking fashion to form a double-layered roof–floor-like framework, with an interplanar distance of 3.72 Å (Figure 1c and Supporting Information Figure S2a). Other phenyl and anthracyl rings formed T-shaped π-stacking with strong C–H⋯π bonds in the range of 2.89–3.53 Å ( Supporting Information Figures S2a and S2b). The two adjacent APT molecules adopted a back-to-back arrangement to form the porous wall, in which two propeller-like wedges cross-interlocked by π⋯π and C–H⋯π interactions with bonds between 3.87 and 2.43 Å, respectively (Figure 1c and Supporting Information Figure S2c). These planar π-conjugated groups stack up to create unique 1D nanopores with an aromatic passageway (Figures 1c and 1d and Supporting Information Figure S2d). Note that this nanopore was irregular and had two kinds of narrow pore apertures, defined as Mode-I and Mode-II, respectively ( Supporting Information Figure S3). As in Mode-I, the pore aperture was surrounded by four APT molecules. The shortest H⋯H distance (d(H⋯H)1) from two phenylenes on the roof–floor component is ca. 6.25 Å (regardless of the van der Waals radii), while the corresponding centroid⋯centroid separation (d(π⋯π)1) between anthracene planes is measured as 10.61 Å. For Mode-II, however, the pore aperture was surrounded by another four adjacent APT molecules. The shortest centroid⋯centroid separation (d(π⋯π)2) between anthracene planes of APT molecules from two up–down roof–floor components, recorded as 9.55 Å, and the H⋯H distance (d(H⋯H)2) from anthracene rings on the left-right porous wall of ∼6.55 Å. Considering that the solvent-accessible volume of 1 was 10.6%, the porosity of 1 was investigated. The activated 1 shows no adsorption for N2 at 77 K but displayed an analogous type IV sorption isotherm for CO2 at 195 K (the adsorption value was 63.7 cm3·g−1; Supporting Information Figure S4). There was a gate-opening action in the high relative pressure region (P/P0 = 0.85), indicating that 1 had intrinsic flexibility (Figure 2b and Supporting Information Figure S4b).47,48 The calculated Brunauer–Emmett–Teller (BET) surface area was 150.6 m2·g−1, and the pore-size distribution estimated using the Horvath–Kawazoe cumulative pore volume plot was unimodal with a maximum peak at 5.7 Å ( Supporting Information Figure S5), which was in excellent agreement with the diameter of the 1D nanopore observed in the structure. In addition, further research indicated that activated 1 exhibited low adsorption capacity for light hydrocarbons at room temperature ( Supporting Information Figure S6). Figure 1 | (a) Molecular structure of APT. (b) Crystallographic configuration of APT in the asymmetric unit. The aromatic rings are colored in red, orange, and yellow to distinguish them in different planes. (c) Space-filling representation of the roof–floor-like formation and the porous wall. (d) The 1D rectangular box-shaped nanopore along the crystallographic b axis of 1. The shape of the cavities shows yellow (outside) and green (inside). Color code: gray, C; lavender, H. Download figure Download PowerPoint Stability and flexibility of 1 Previous studies show that parallel-displaced and T-shaped are the two kinds of stable π-stacking modes.36,37,49,50 When the π system was designed as anthracene, the π–π stacking interaction energy reached a significant −7.726 kcal·mol−1.49 This energy would dramatically increase as a dimer, trimer, and multimer, formed between molecules, which means better stability and durability than most hydrogen bonds.46,51,52 Consequently, crystalline 1 displayed prominent thermal and chemical stability. According to thermogravimetric analysis (TGA), 1 is thermally stable up to 450 °C under nitrogen ( Supporting Information Figure S7). To investigate the chemical stability of 1, we exposed its crystals to different organic solvents for 1 week, including methanol, ethanol, acetone (dimethyl ketone; DMK), n-hexane, cyclohexane (CYH), acetonitrile, diethyl ether, and mixed solvents such as toluene (TOL)–ethanol, dioxane (DOX)–ethanol, and dimethylformamide–ethanol (1:10, v/v). The 1 well maintained its crystallinity ( Supporting Information Figure S8). Even in harsh chemical environments such as boiling water, HCl (1 M), NaOH (1 M), and trifluoroacetyl (TFA) (1 M in ethanol) for 1 week, no damage was found to the crystals, evidenced by the imperceptible change of the peaks in their powder XRD (PXRD) patterns (Figure 2a). After these rigorous treatments, the gas adsorption experiments showed an only slight drop in CO2 uptake (the adsorption values were recorded as 58.5, 57.6, 55.1, and 53.0 cm3·g−1, corresponding to boiling water, HCl, TFA, and NaOH treatment, respectively), indicating that 1 could retain its structural stability and permanent porosity (Figure 2b). Such structural robustness is comparable to other known ultrastable crystalline porous materials such as COFs (PAE-COFs),53 HOFs (Pyopen, HOF-TCBP, and PFC-1),51,54,55 and surpasses the performance of typical MOFs (MOF-5, HKUST-1, and ZIF-8).56–58 Furthermore, the disordered solvents in the as-synthesized 1 could be removed readily through an ethanol exchange process. No crystal degradation occurred during this process, and the afforded guest-free crystals were abbreviated as 1-None. Interestingly, the 1-None also exhibited dynamic temperature-dependent structural changes. From 100 to 353 K, a, b, and c axes, as well as the cell volume V all demonstrated positive thermal expansion with slight increases by 0.4%, 1.9%, 0.5%, and 3.0%, respectively ( Supporting Information Figure S9 and Tables S1 and S5) when cooled from 353 to 100 K. The unit cell parameters recovered in the SC-SC transformation, which indicated reversibility of the thermal expansion. Figure 2 | (a) PXRD patterns of as-synthesized 1 and treated under different conditions. (b) CO2 adsorption isotherms (195 K) of as-synthesized 1 and the samples treated with boiling water, 1 M HCl, NaOH, and TFA solution. Download figure Download PowerPoint Crystalline sponge for determination of small molecules It was expected that the low symmetry, high thermal and chemical stability of the structure, particular π-stacking fashion, and the appropriate flexibility of pore would make 1-None a promising crystalline sponge candidate. As proof, 14 chemicals with different functional groups from hydrophilic to hydrophobic, from aliphatic to aromatic, including DMK, THF, DOX, n-hexane, CYH, methylcyclohexane (MCYH), chlorocyclohexane (ClCYH), BZ, chlorobenzene (ClBZ), TOL, p-xylene (PX), ethylbenzene (EtBZ), aniline (Aniline), and benzaldehyde (PHCHO), were chosen as target guest molecules. The crystals of 1-None were immersed in ethanol containing each of the above chemicals at room temperature for 5 days (ethanol/guests in 10∶1, v/v). The monitored crystallization was mounted directly onto an X-ray diffractometer to collect the diffraction data after the solvent-soaking process. Thus, the crystal structures obtained in an SC-SC transformation showed that the guests had been trapped in the flexible cavity, leading to the formation of [email protected], [email protected], [email protected], C6H14@1, [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], and [email protected], respectively (Figure 3 and Table 1, as well as Supporting Information Figures S10 and S11 and Tables S1–S4). XRD analysis suggested that all the crystals remained in the P21/c space group even though the guest molecules vary. The phase purity of all compounds was evaluated using PXRD ( Supporting Information Figures S12–S25). Interestingly, all the hydrophilic molecules entered the pores along the Mode-I direction (Figures 3a–3c) and formed C–H⋯π interactions with corresponding anthracenes on the roof–floor components. For instance, in [email protected], the THF molecules slid into the pores with their polar ends orientated precisely to the H atoms on the phenylene rings of the roof–floor-like porous wall and formed strong C–H⋯O hydrogen bonds recorded as 2.84 Å. Furthermore, the five-membered heterocycles kept an almost planar configuration. The H atoms on the heterocycle pointed to the centroid of anthracene rings on the roof–floor components and formed strong C–H⋯π interactions (2.90 and 3.23 Å) (Figure 3b). These multiple interactions guided the THF molecules to be captured in a given position in the 1D channels and enabled the corresponding configurations to be well-defined. In contrast, the aliphatic and aromatic molecules entered the pores along the Mode-II direction (Figures 3d–3n) and formed multiple C–H⋯π interactions with the two anthracenes in both upper and lower positions on the roof–floor structure. The straight-chain n-hexane molecules were fixed in the microchannel by multiple C–H⋯π interactions (3.24 and 3.25 Å, respectively) deriving from methylene to roof–floor components (Figure 3d). CYH is the most widespread ring in natural compounds and holds a special place in the history of organic chemistry.59–61 It is well known that the CYH molecule has the ability to switch between several 3-Dimensional conformations and that only the chair and twist-boat conformations can be isolated into respective pure forms due to their lower energies.62–64 Interestingly, the CYH molecules were in the pores by strong C–H⋯π interactions (2.90 and Å, respectively) and adopted the most stable chair (Figure CYH conformations in which the adopted an the chair CYH is when the hydrogen atoms were by a group or (Figures and For the molecule is in the through C–H⋯π The H atoms from methylene were almost to the anthracene planes of the roof–floor strong C–H⋯π interactions recorded as and Å, and stable interactions between the host framework and guests the chair of CYH and its which was highly to crystal structure determination. Figure 3 | structures with guest molecules captured in 1 are indicated with thermal The of the host frameworks are with a interactions between the molecules and the roof–floor components are indicated with and DMK, THF, DOX, n-hexane, CYH, BZ, TOL, and molecules captured in (a) [email protected], (b) [email protected], (c) [email protected], (d) C6H14@1, [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], and [email protected], Color for guest C; red, Download figure Download PowerPoint Table 1 | of Crystalline Sponge Crystal and Structure Results a b c V 1 P21/c 1-None P21/c [email protected] P21/c [email protected] P21/c [email protected] P21/c P21/c [email protected] P21/c [email protected] P21/c [email protected] P21/c [email protected] P21/c [email protected] P21/c [email protected] P21/c [email protected] P21/c [email protected] P21/c [email protected] P21/c [email protected] P21/c As in Figure the frameworks [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], and [email protected] contain a aromatic guest in an asymmetric and the guest molecules into the pores along the Mode-II [email protected] as an instance, the molecules are in the pore and almost the dihedral and respectively) with anthracyl on the roof–floor components and the pore wall. Such a unique position guests and host framework to strong interactions through C–H⋯π Crystal structure analysis demonstrated that the H atoms in the guest aromatic rings pointed the centroid of the roof–floor anthracene planes with strong C–H⋯π interactions from to Å) (Figures the H atoms in the anthracyl ring from the pore wall also exhibited C–H⋯π interactions with the guest aromatic rings from to which allowed the aromatic molecules to be precisely and in the pore (Figures Furthermore, it was to that groups such as and were exposed to the porous According to different guests, the H⋯H and the π⋯π of the roof–floor structure showed dynamic and expansion. As in Supporting Information Table the hydrophilic structures of [email protected], [email protected], and [email protected] showed small within For a of the straight-chain n-hexane the flexible porous structure adopted with the significant recorded as and For the molecules, a large expansion of the H⋯H of occurred to guests. in [email protected] and [email protected], the up to Å with an expansion of In contrast, the when 1 aromatic guests. for [email protected], the other frameworks [email protected], [email protected], [email protected], [email protected], [email protected], and [email protected] all exhibited prominent with between and For with [email protected], the separation to Å with a maximum of ( Supporting Information Table S6). The and showed a to to encapsulate the guests This excellent dynamic ability the 1-None as a promising crystalline sponge However, it is a that the of 1-None the of guest molecules, including anthracene and into the Therefore, it is to porous organic molecular frameworks with for more with the crystalline sponge of the crystalline sponge host–guest Such crystalline sponge method can also be with In CDCl3, 1-None displayed two at 8.57 and 8.24 two at and 7.66 two as a at 8.11 and two from different H of the anthracyl were at 7.53 and respectively (Figure and Supporting Information Figure The of [email protected] showed a single peak of at The of the peak area of H with the area of 1-None H that a molecule of in each asymmetric unit (Figure and Supporting Information Figure with the of the structural analysis. Other frameworks such as [email protected], [email protected], [email protected], [email protected], [email protected], and [email protected] were all detected by the corresponding with an guest molecule in the asymmetric unit (Figures and Supporting Information Figures The aliphatic guests were defined in the with the host–guest of ( Supporting Information Figures molecules were also found in the of [email protected], [email protected], and [email protected] ( Supporting Information Figures the of the peak of H and THF H was a due to the during the the "crystalline sponge" effect of 1-None was Figure 4 | of samples in (a) (b) [email protected], (c) [email protected], (d) [email protected], [email